Today numerous different thermoplastic polymers are commercially used because each has a combination of physical characteristics that make it well-suited for particular applications. In many instances, it is an undesirable characteristic that prevents a thermoplastic polymer from fully participating in some applications. For example, thermoplastic polyester has a good combination of strength, dimensional stability, and cost, but surface related problems such as adhesion, UV resistance, abrasion resistance, etc. inhibit its potential.
Polyester fiber replaced nylon fiber in passenger tire bodies because polyester was more dimensionally stable and hence the resulting tires did not exhibit objectionable flat-spotting. Due to its combination of strength and dimensional stability, polyester remained the preferred tire body reinforcement as passenger tires went from bias to radial constructions. Polyester""s poor adhesion to rubber was overcome by using aggressive adhesion promoters in combination with higher temperatures and residence times in down-stream dipping and heat-setting operations. See for example U.S. Pat. No. 4,300,972 and W. G. Perkins, xe2x80x9cComplexities in PET Tire Yarn Processing and Characteristicsxe2x80x9d, International Fiber Journal 42 (September 1987) and R. Iyengar, xe2x80x9cAdhesion of tire cordsxe2x80x94the total picturexe2x80x9d, RUBBER WORLD 197(2) 24(1987). This added cost from reduced output, higher energy input, and control equipment for containing the added environmentally unfriendly chemicals makes the conventionally used dip additives for adhesive promotion unattractive. Even with its objectionable flat-spotting, nylon is preferred over polyester in tire cap plies due to at least partly its inherently better hot adhesion. Thus, an article having a combination of polyester physical properties with a xe2x80x9cnylon-likexe2x80x9d surface would be highly desirable for tire applications.
Reduced friction during polymeric fiber processing and abrasion during end-use are also currently addressed by topically applying a finish during fiber spinning and drawing. These finishes are applied as solutions or emulsions and hence have the difficulties discussed above. Similarly, a polymeric fiber having a permanent outer layer exhibiting low friction and/or abrasion resistance would be a highly desirable solution.
Ultraviolet (xe2x80x9cUVxe2x80x9d) resistance is currently improved by introducing UV xe2x80x9cscreensxe2x80x9d via topically applied coatings or additives to the polymer melt. Coatings lack permanency. Uniform addition to the fiber adds extra cost, but little benefit from xe2x80x9cscreensxe2x80x9d located well below the surface. Preferential location of a UV stabilizer in a permanent layer near the surface would be a highly desirable solution.
A seldom used approach for fiber production has been the incorporation of low molecular weight additives which xe2x80x9cbloomxe2x80x9d to the surface during fiber extrusion, fiber drawing, and/or during subsequent use. This approach avoids the environmental issues associated with the above approaches, but it does not produce the sought-after permanent surface for applications where abrasion or shear at the fiber/matrix surface is present. Blooming is disclosed in U.S. Pat. No. 3,973,068 wherein a surfactant is added to polyolefin and the surfactant migrates to the fiber surface and reduces secondary bonding. U.S. Pat. No. 4,640,962 teaches a silicone-sheathed polyester fiber wherein (1) the silicone is added from 0.1 to 10 weight percent to the polyester, (2) per column 8, lines 24-27, microdomains (preferably less than 1 micron) are formed xe2x80x9cso that the endgroups of all of the polysiloxane block polymer have an opportunity to condense with the polyester,xe2x80x9d and (3) the microdomain migrates to the fiber surface during spinning and drawing. Per column 8, lines 50-53, xe2x80x9csurprisingly, the migration of the silicone domains has been found to continue during drawing, including cold drawing . . . .xe2x80x9d The low inherent surface energy for the polysiloxane and resulting driving force to occupy a surface geometry was sufficient for the formation of a xe2x80x9csilicone sheath.xe2x80x9d
Similarly, U.S. Pat. No. 5,069,970 teaches the use of low surface energy organic polymers to preferentially locate at the surface of PET fibers for use as high capacity air filter fibers. Polypropylene and poly(methylpentene) are the only additives in the patent examples. A wider range of polymers is suggested in the patent text, but all the polymers are inherently inert and incapable of thermally reacting with PET.
In contrast in the present invention, additives with higher surface energies are preferentially located at the article surface. Therefore, although not wishing to be bound by theory, it is believed that the mechanism for this invention is fundamentally different in a manner that provides much greater opportunity for surface engineering. Furthermore, the surprising ability to incorporate reactive groupings such as amides, esters, unsaturated olefins, etc. into melt formed articles while maintaining the base thermoplastic properties and achieving the desired propensity for bonding is a further differentiating feature.
This invention relates to heterogeneous or immiscible blends of two or more polymers. The Encyclopedia of Polymer Science and Engineering 12, 403-424 (1988) reviews the various methods for establishing blend heterogeneity. Thermal (DSC and DTA), Dynamical Mechanical, and Microscopy (optical, TEM, SEM) methods are particularly useful. As general guide, blend miscibility can be estimated using solubility parameters (see M. M. Coleman et. al. Polymer 31, 1187 (1990)). A lower solubility parameter signifies a lower surface energy and hence a greater propensity to preferentially locate at the article surface. For this invention, solubility parameters are defined in terms of the values calculated using Coleman""s methodology and his constitutive molar volumes and attraction constants. When using copolymeric additives, the relative abundance of the constitutive functional units is proportionated in accordance with their mole fraction. Therefore, the calculated solubility parameter for a Nylon 6/Nylon 11 copolymer with 33 mole % Nylon 6 would be calculated as follows:
{0.33[5xc3x97132+405]+0.67[10xc3x97132+405]}/{0.33[5xc3x9716.5+19.2]+0.67[10xc3x9716.5+19.2]}=9.6 
End-capping agents would also be included in the analysis and would also be proportionated on a mole fraction basis. For ease of reference these calculated solubility parameters will be referred to as xe2x80x9cCSPxe2x80x9d values.
The CSP value is 7.4 for both the polypropylene and poly(methyl pentene) exemplified in above U.S. Pat. No. 5,069,970. In the broadest patent claim, polybutylene has the highest CSP value at 7.6. This patent teaches the most preferred polyolefins have high molecular weight in the 50,000 to 500,000 range.
The desire to have a certain base fiber for mechanical properties and cost and a permanent outer layer or sheath with markedly different physical characteristics has been a major driving force for bi-component spinning. While this approach does provide the basic fiber structures desired as solutions for the above problems, it has disadvantages. First, additional equipment is required including an additional extruder to introduce the sheath polymer and sophisticated spinnerettes to channel that sheath polymer to extrude it as the outer layer of the individual filaments. For current day multi-end processes, these spinnerettes can have 1000 extrusion holes. Second, it is quite difficult to make sheath-core filaments where the sheath is present at 5% or less of the fiber volume. Both factors represent significant added cost in terms of added equipment, excess sheath weight, and scrap arising from added process control difficulties. Also, the melt viscosity of the sheath and core must be similar in order to be spinnable. Finally and possibly most importantly, poor adhesion between the fiber core and the sheath often occurs resulting in a propensity for failure via delamination at the sheath-core interface. This would be particularly problematic in tire applications where there is high strain flexing of the filaments.
U.S. Pat. No. 5,468,555 claims sheath/core yarns with uniform sheath dimensions throughout the yarn bundle. This patent teaches the extrusion process for making these more uniform filaments and discloses suitable sheath polymers for polyethylene terephthalate (xe2x80x9cPETxe2x80x9d) to be nylon66, polyether sulphone, polyimide, polytetrafluoroethylene, polyphenylene sulfide, and polypropylene. While this list covers a wide range of polymers, only PET with a high molecular nylon 66 sheath is exemplified. This nylon66 sheath is present from 6.9 to 15.2% (by volume). Some yarns were converted into dipped tire cord, but no adhesion data was provided. Comparative Examples BF1-9 in this disclosure cover PET with Nylon 6 sheath ranging from 1-20%. SEM micrographs for the yarns with 1 and 5% sheath levels show substantial delamination even in the undrawn state. The 20% sheath samples did not exhibit this delamination in the undrawn or drawn yarns. However, adhesion for the treated cord was well-below that for a nylon surface due to premature delamination during the adhesion test. Such delamination occurs because there is no chemical or physical reaction occurs between the core and sheath materials. This propensity for delamination is supported by U.S. Pat. No. 5,582,913 and European Patent 471,088A1 which teach in their background sections that separation of a PET core from a nylon 6 sheath is a common problem and shows in FIG. 2, that the core and sheath break separately thus reducing the fiber strength. Although U.S. Pat. No. 5,582,913 teaches that delamination during tensile testing can be reduced by introducing a nylon 6/nylon 12 copolymer to improve the compatibility between the sheath and core, no data is presented to show that the adhesion between the sheath and core is sufficient to provide good adhesion of the resulting fibers to rubber. Furthermore, the above-mentioned comparative fibers with 1-20% nylon6 sheath showed increased sheath volume fraction reduced delamination, but adhesion performance was still inferior. Since the examples in U.S. Pat. No. 5,582,913 have 50% sheath, the absence of apparent delamination during the tensile test is no indication of adequate adhesion performance at 10% or less sheath levels. European Patent 471,088A1 requires a complicated series of extruders and special spinnerets to achieve protrusions between the sheath and core. See also U.S. Pat. No. 4,859,759. The following Table lists other references teaching higher sheath percentages:
Another approach for fiber production is blends as disclosed in U.S. Pat. No. 4,066,587 wherein a polyamide (formed from a long-chain dibasic acid containing at least 18 carbon atoms and a diamine) is added at 0.01 to 20 weight percent to polyester. Example VII represents the closest art in U.S. Pat. No. 4,006,587 and it is outside this invention because the polyamide is not end-capped and hence will react with the polyester thereby lowering its viscosity and impeding movement of the additive to the fiber surface. See Japan Patent Publication 4336-1971 (published Feb. 3, 1971) teaching melt spinning polyester at 10-40 weight percent in polyamide. The following Table lists other blends.
Another approach for fiber production is copolymers as disclosed in commonly assigned European Patent 703,938B1. The starting polymers in the melt react together to form copolymers.
Surface modified polyamides are known.
Thus, the art needs a fiber with polyester physical properties and a permanent outer layer providing functionality not normally provided by or associated with polyester. Desirable outer layers include a nylon-like surface having good adhesion between the two polymeric surfaces and polyolefin-like surface having improved lubricity and abrasion resistance. The art also needs a process for making such a fiber wherein the process avoids the deficiencies and problems associated with sheath-core fiber.
This invention responds to the need for thermoplastic articles which maintain their inherent mechanical properties and cost structure yet have a permanent outer surface that has selectively varied chemical functionality. The major benefit is to overcome inherent deficiencies related to: (1) article incompatibility with different composite matrices, (2) inadequate environmental stability (light, chemical, etc.), and (3) general surface-related end-use characteristics such as poor abrasion resistance, excessive friction, etc. The present invention describes the additives, the fiber-making process, and the resulting novel articles. The process involves
(a) adding a substantially organic molten component with CSP value of at least 8 to a molten thermoplastic polymer and mixing to substantially uniformly disperse the molten component in the molten thermoplastic polymer and form a heterogeneous blend wherein
(i) the melt viscosity of the molten component is substantially less than the melt viscosity of the molten thermoplastic polymer; and
(ii) the amount of the molten component in the molten thermoplastic polymer is up to about ten percent by weight based on the heterogeneous blend; and
(b) melt processing the heterogeneous blend wherein the molten component preferentially locates near the surface of the molten thermoplastic polymer and substantially no chemical reaction occurs between the molten component and the molten thermoplastic polymer.
The selection criteria for this additive polymer or molten component are fourfold: (1) the additive has the sought-after characteristics desired for the article surface, (2) the additive forms a well-dispersed heterogeneous blend structure with the thermoplastic polymer matrix, (3) there is substantially no reaction between the additive and base polymers during melt formation, and (4) the additive""s melt viscosity at the melt formation temperature is substantially lower than that for the base polymer. We believe, but are not bound, that the mechanism wherein the tendency for the low viscosity component to locate at the ultra high shear region adjacent to apparatus stationary surfaces is a primary driving force for it to preferentially locate at the article surface. The propensity for the low viscosity additive to xe2x80x9cwetxe2x80x9d the extrusion surface may also play a role. The term xe2x80x9csubstantially lowerxe2x80x9d viscosity means the ratio of the base molten polymer to the molten additive at the melt processing temperature is at least about 1.5/1, preferably at least about 3/1, and most preferably at least about 10/1. The term xe2x80x9csubstantially organicxe2x80x9d means 85% of the polymer is based on organic molecules as exemplified in the present invention by polyethylene, polybutadiene, and polyamides.
We responded to the need for additives meeting the above criteria and have developed surface activating polyamide (Formula (I)) and polyolefin (Formula (II)) additives.
Formula (I) is an end-capped polyamide or copolyamide of moderate molecular weight comprising one or more of any of the following units
(a) xe2x80x94[xe2x80x94NHxe2x80x94(CH2)xxe2x80x94C(xe2x95x90O)xe2x80x94]xe2x80x94 where x=3-30; or
(b) xe2x80x94[xe2x80x94NHxe2x80x94R1xe2x80x94NHxe2x80x94C(xe2x95x90O)xe2x80x94R2xe2x80x94C(xe2x95x90O)xe2x80x94]xe2x80x94 where R1 and R2 are independently selected from
(i) xe2x80x94(CH2)Yxe2x80x94 or where Y=1-30; or
(ii) xe2x80x94CH2xe2x80x94(CH2xe2x80x94Oxe2x80x94CH2)Zxe2x80x94CH2xe2x80x94 where Z=1-30; or
(iii) for R2, hydrocarbon component comprising acyclic, monocyclic, bicyclic, and aromatic units and are partially or fully hydrogenated as long as the resulting additive has a lower melting point than the thermoplastic polymer
and the polyamide or copolyamide is terminated to reduce the free carboxyl and amine end-groups. The terminating agents have functional groups capable of reacting with the free carboxyl or amine end-groups and consist of a substituted or unsubstituted aliphatic or aromatic groups having from two to 100 carbon atoms.
Formula (II) is derived (prepared) from an end-capped polyolefin of moderate molecular weight and contains any of the following units
(a) xe2x80x94[xe2x80x94NHxe2x80x94R3xe2x80x94NHxe2x80x94]xe2x80x94
(b) xe2x80x94[xe2x80x94C(xe2x95x90O)xe2x80x94R4xe2x80x94C(xe2x95x90O)xe2x80x94]xe2x80x94
where R3 and R4 are polyolefin residues with degree of polymerization up to 250.
The terminating agents are similar to those described for Formula (I).
These additives are preferably used in thermoplastic polymers and more preferably in polyester.